A noise suppressing technology is known as a signal processing technology of partially or completely suppressing noise in a noisy signal (a signal containing a mixture of noise and a target signal) and outputting an enhanced signal (a signal obtained by enhancing the target signal). For example, a noise suppressor is a system that suppresses noise mixed in a target audio signal. The noise suppressor is used in various audio terminals such as mobile phones.
Concerning technologies of this type, patent literature 1 discloses a method of suppressing noise by multiplying an input signal by a suppression coefficient less than 1. Patent literature 2 discloses a method of suppressing noise by directly subtracting estimated noise from a noisy signal. However, the techniques described in patent literatures 1 and 2 include an averaging operation in noise estimation. Therefore, the noise that occurs in an unexpected fashion such as impact sound cannot be sufficiently suppressed.
In contrast, non-patent literature 1 discloses a noise suppressing system which detects the impact sound based on flatness of a power spectrum of a noisy signal and the increment from the past. A system disclosed in non-patent literature 1 estimates background noise when the impact sound is not detected in a non-voice section. The system disclosed in non-patent literature 1 suppresses the impact sound by replacing the noisy signal with the estimate of the background noise when the impact sound is detected in the non-voice section and updates an impact sound estimate by using a difference between the noisy signal and the background noise. The system disclosed in non-patent literature 1 suppresses the impact sound by subtracting the impact sound estimate from the noisy signal when the impact sound is detected in a voice section.